1. Field of the Invention
The present invention relates to distance measuring apparatus for measuring the distance to an object to be measured and, more particularly, to distance measuring apparatus of an active type suitably applicable to cameras and others.
2. Related Background Art
The distance measuring apparatus of the active type used in the cameras and others is configured to project light from an infrared emitting diode (which will be referred hereinafter as xe2x80x9cIREDxe2x80x9d) toward an object to be measured, receive reflection of the projected light by a position sensitive device (which will be referred to hereinafter as xe2x80x9cPSDxe2x80x9d), arithmetically process signals outputted from this PSD by a signal processing circuit and an arithmetic circuit to provide distance information, and determine the distance to the object by a CPU. Since distance measurement with only one light projection can produce an error, it is common practice to perform a plurality of light projections to obtain a plurality of distance information and integrate the plurality of distance information by an integrating circuit to average the information.
The conventionally known distance measuring devices of this active type include those described in Japanese Patent Applications Laid-open No. H08-94919 and No. H08-94920. FIG. 22 is a block diagram of the distance measuring apparatus described in these applications, which will be referred to as the distance measuring apparatus according to the first prior art.
In the distance measuring apparatus shown in FIG. 22, a driver 112 drives the IRED 114 under control of CPU 110 to make it output infrared light, and the infrared light is projected through a projection lens (not shown) toward an object to be measured. The infrared light reflected by the object is converged through a receiving lens (not shown) on the PSD 116, and the PSD 116 outputs two signals I1 and I2 according to a position where the reflection of the infrared light is received. A first signal processing circuit 118 removes a stationary light component of noise included in the signal I1 and a second signal processing circuit 120 a stationary light component of noise included in the signal I2.
An arithmetic circuit 132 calculates an output ratio (I1/(I1+I2)), based on the signals I1 and I2 obtained after the removal of the stationary light components, and outputs an output ratio signal according to the distance to the object. An integrating circuit 134 integrates a multiplicity of output ratio signals outputted in this way from the arithmetic circuit 132 to improve an S/N ratio. This integrating circuit 134 outputs a signal (hereinafter referred to as xe2x80x9cAF signalxe2x80x9d) according to the distance to the object. Then the CPU 110 executes a predetermined operation to obtain a distance signal, based on the AF signal outputted from the integrating circuit 134, and controls a lens driving circuit 136, based on this distance signal, to move a lens 138 to an in-focus position.
FIG. 23 is a drawing showing a relation between the AF signal outputted from the integrating circuit 134 of the first prior art and the distance to the object. In the graph shown in this drawing, the abscissa represents the inverse (1/L) of the distance L to the object and the ordinate the output ratio (I1/(I1+I2)) or the AF signal. As shown in this figure, the output ratio is substantially in a linear relation to the inverse (1/L) of the distance L in the range not more than a certain distance L4. Thus the output ratio becomes smaller as the distance L increases (or as 1/L decreases). In the range not less than the distance L4, however, the influence of the noise component becomes larger to the contrary with increase in the distance L. When the noise component is denoted by In (Inxe2x89xa70), the output ratio is given by (I1+In)/(I1+In+I2+In). In the range larger than the distance L4, the output ratio varies so as to increase (i.e., toward the output ratio of 50%). In addition, since In occurs at random, the output ratio becomes unstable depending upon measurement conditions. The reason is that with increase in the distance L the intensity of the reflected light received by the PSD 116 becomes smaller and the noise component In becomes relatively larger. With occurrence of this phenomenon, the distance L to the object cannot be uniquely determined from the output ratio.
For this reason, as shown in FIG. 24, a clamping circuit 130 is interposed between the second signal processing circuit 120 and the arithmetic circuit 132 to compare the far signal I2 outputted from the second signal processor 120, with a clamp signal Ic and output the clamp signal Ic when the far signal I2 is smaller than the clamp signal Ic. Even in this structure, however, the distance output is fixed at a certain distance on the far side, as shown in FIG. 27 described hereinafter, and there occurs great deviation from designed values.
The distance measuring devices giving a solution to this problem include those described below. FIG. 25 is a block diagram of the distance measuring apparatus according to the second prior art. This figure shows only the structure on the photoreceptive side. In the distance measuring apparatus shown in this figure, the signals I1 and I2 outputted from the PSD 140 are supplied to respective stationary light removing circuits 142 and 144 to remove the stationary light component therefrom and thereafter the resultant signals are given to respective arithmetic circuits 146 and 148. The arithmetic circuit 146 performs an operation of I1/(I1+I2) to obtain an output ratio, based on the signals I1 and I2 after the removal of the stationary light components, and the integrating circuit 150 integrates the output ratio. On the other hand, the arithmetic circuit 148 performs an operation of I1+I2 to obtain the quantity of light and the integrating circuit 152 integrates the quantity of light. Then a selection part 160 selects either one of the output ratio and the quantity of light and calculates the distance to the object, based on the selected. The selection part 160 is a process in the CPU.
FIG. 26 is a block diagram of the distance measuring apparatus according to the third prior art. This figure also shows only the structure on the photoreceptive side. In the distance measuring apparatus shown in this figure, the signals I1 and I2 outputted from the PSD 170 are supplied to respective stationary light removing circuits 172 and 174 to remove the stationary light component therefrom and thereafter either of the resultant signals is given to one end of switch 176. Under control of the CPU, this switch 176 supplies either of the outputs from the stationary light removing circuits 172 and 174 into the integrating circuit 178. The integrating circuit 178 integrates either one of the input signals I1 and I2. An arithmetic part 180 executes an operation of I1/(I1+I2) to obtain an output ratio, based on the integration result, while an arithmetic part 182 does an operation of I1+I2 to obtain the quantity of light. Then a selection part 184 selects either one of the output ratio and the quantity of light and calculates the distance to the object, based thereon. The arithmetic parts 180, 182 and the selection part 184 are processes in the CPU.
These distance measuring devices (FIG. 25 and FIG. 26) according to the second and third prior arts are constructed both to calculate the distance L, based on the output ratio (I1/(I1+I2)), when the distance L to the object is small, but calculate the distance L, based on the light quantity (I1+I2), when the distance L is large, whereby the distance L can be uniquely determined.
As described above, the distance measuring devices according to the second and third prior arts (FIG. 25 and FIG. 26) are the apparatus that can give a solution to the problem of the distance measuring apparatus according to the first prior art (FIG. 22 and FIG. 24). However, the distance measuring apparatus of the second prior art (FIG. 25) needs to use the two sets of arithmetic circuits and integrating circuits and thus involves a problem of increase in the circuit scale and, in turn, increase of cost, as compared with the distance measuring apparatus of the first prior art (FIG. 22, FIG. 24). On the other hand, the distance measuring apparatus of the third prior art (FIG. 26) can be constructed in the smaller circuit scale, but does not allow simultaneous detection of both the signals I1 and I2 from the PSD 170 and thus requires double time for obtaining the distance L at the S/N ratio equivalent to that in the distance measuring apparatus of the second prior art (FIG. 25).
The above distance measuring devices of the prior arts all are designed to operate in good order when the reflectance of the object (subject) for the infrared light outputted from IRED is a standard value. When the reflectance of the object is low, the PSD outputs small values of the signals I1 and I2 and the apparatus can fail to obtain an accurate distance value. This problem appears significant, particularly, when the distance to the object is large. This will be described using the calculation results shown in FIG. 27 to FIG. 30.
FIG. 27 is a graph showing a relation between the distance and the distance signal obtained by the distance measuring apparatus of the first prior art when the reflectance of the object is the standard value, 36%. FIG. 28 is a graph showing a relation between the distance and the distance signal obtained by the distance measuring apparatus of the first prior art when the reflectance of the object is as low as 9%. FIG. 29 is a graph showing a relation between the distance and the distance signal obtained by the distance measuring apparatus of the second and third prior arts when the reflectance of the object is the standard value, 36%. FIG. 30 is a graph showing a relation between the distance and the distance signal obtained by the distance measuring apparatus of the second and third prior arts when the reflectance of the object is as low as 9%. In these figures, two dashed lines parallel to each other indicate the tolerance of measurement error.
In the distance measuring apparatus of the first prior art (FIG. 24), when the reflectance of the object is the standard value of 36%, as shown in FIG. 27, the distance signal always falls within the tolerance of measurement error, but the distance signal barely falls within the tolerance at some distances. On the other hand, when the reflectance of the object is as low as 9%, as shown in FIG. 28, the distance signal is off the tolerance of measurement error at certain distances. In the case of the distance measuring apparatus of the second and third prior arts, when the reflectance of the object is the standard value of 36%, as shown in FIG. 29, the distance signal is always within the tolerance of measurement error and this is an improvement as compared with the one shown in FIG. 24. On the other hand, when the reflectance of the object is as low as 9%, as shown in FIG. 30, the distance signal is off the tolerance of measurement error at certain distances, and this is similar to that shown in FIG. 28.
As described above, when the reflectance of the object is low, the distance signal can be off the tolerance of measurement error, depending upon the distance, so that the distance measurement (ranging) accuracy can become degraded. In order to solve this problem, it is thus conceivable to provide the clamping circuit for outputting the clamp signal Ic when the far signal I2 outputted from the second signal processing circuit 120 is smaller than the clamp signal Ic, between the second signal processor 120 and the arithmetic circuit 132 and set the level of this clamp signal Ic to a small value, in the distance measuring apparatus of the first prior art (FIG. 24).
FIG. 31 is a graph showing a relation between the distance and the distance signal obtained by the distance measuring apparatus of the first prior art when the level of the clamp signal Ic is set to a small value and when the reflectance of the object is the standard value of 36%. FIG. 32 is a graph showing a relation between the distance and the distance signal obtained by the distance measuring apparatus of the first prior art when the level of the clamp signal Ic is set to a small value and when the reflectance of the object is as low as 9%. As shown in FIG. 31, when the level of the clamp signal Ic is low and when the reflectance of the object is the standard value, the distance signal always falls within the tolerance of measurement error and, in addition, this is an improvement from that shown in FIG. 27. On the other hand, when the level of the clamp signal Ic is low and when the reflectance of the object is low, as shown in FIG. 32, the distance signal barely drops within the tolerance at some distances, while the distance signal is always within the tolerance of measurement error.
However, even if the level of the clamp signal Ic is set to the small value, when the luminance of the ambient light is relatively high, another problem will arise as follows. FIG. 33 is a graph showing a relation between the distance and the distance signal obtained by the distance measuring apparatus of the first prior art when the level of the clamp signal Ic is low, when the luminance of the ambient light is high, and when the reflectance of the object is the standard value of 36%. FIG. 34 is a graph showing a relation between the distance and the distance signal obtained by the distance measuring apparatus of the first prior art when the level of the clamp signal Ic is low, when the luminance of the ambient light is high, and when the reflectance of the object is as low as 9%. As shown in these figures, when the ambient light luminance is high, the tolerance of measurement error becomes wider, but, in spite thereof, when the reflectance of the object is not only the standard value but also lower than it, the distance signal can be off the tolerance of measurement error, depending upon the distance, so that the ranging accuracy can become degraded. The reason is that at high luminance of the ambient light the first signal processor 118 and the second signal processor 120 can fail to remove the stationary light fully, so as to cause measurement errors.
In order to solve the above problem, it is conceivable to increase the quantity of the light projected from the IRED or to increase the diameters of the projection lens and the receiving lens. FIG. 35 is a graph showing a relation between the distance and the distance signal obtained by the distance measuring apparatus of the first prior art when the quantity of the light projected from the IRED is four times larger and when the reflectance of the object is the standard value of 36%. FIG. 36 is a graph showing a relation between the distance and the distance signal obtained by the distance measuring apparatus of the first prior art when the quantity of the light projected from the IRED is four times larger and when the reflectance of the object is as low as 9%. As shown in these figures, when the reflectance of the object is not only the standard value but also smaller than the standard value, the distance signal is always within the tolerance of measurement error. However, the increase in the quantity of the light projected from the IRED will result in increasing the cost, while the increase in the diameters of the projection lens and the receiving lens will result in increasing the size.
Therefore, the present invention has been accomplished in order to solve the above problem and an object of the invention is to provide distance measuring apparatus that can determine the distance with accuracy even under such circumstances that the reflectance of the object is low and that the distance to the object is large, and that can be constructed without increase in cost and size.
[1] A first aspect of the distance measuring apparatus according to the present invention is a distance measuring apparatus comprising:
light projecting means for projecting light toward an object to be measured;
light receiving means for receiving reflected light of said light projected toward the object, at a reception position on a position sensitive device according to a distance to said object, and for outputting a far signal, which is a value increasing with increase in said distance if quantity of received light is constant, and a near signal, which is a value increasing with decrease in said distance if the quantity of received light is constant;
clamping means for accepting said far signal and comparing the far signal with a level of a clamp signal, wherein said clamping means outputs said far signal if a level of said far signal is not less than the level of said clamp signal, or outputs said clamp signal otherwise;
arithmetic means for calculating a ratio of said near signal and a signal outputted from said clamping means to output an output ratio signal;
integrating means for cumulatively integrating said output ratio signal and outputting an integral signal according to a result of integration; and
control means for controlling each of projection of said light in said light projecting means, the level of said clamp signal in said clamping means, and the total sum of integral time of said output ratio signal in said integrating means, and for detecting a distance value, based on said integral signal outputted from said integrating means,
wherein said control means works as follows:
(a) the control means sets said clamp signal to a first level, and
the control means detects a first distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to a first number;
(b) when said first distance value is a value farther than a first set distance, the control means sets said clamp signal to a second level smaller than said first level, and
the control means detects a second distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to a second number;
(c) when said second distance value is a value nearer than a second set distance, or, when said second distance value is not a value nearer than the second set distance and when a difference of said second distance value from said first distance value is smaller than a first set value,
the control means sets said clamp signal to said first level,
the control means detects a third distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to a third number, and
the control means calculates the distance to said object, based on the sum of said first distance value and said third distance value;
(d) when said second distance value is not a value nearer than the second set distance and when the difference of said second distance value from said first distance value is not smaller than the first set value,
the control means sets said clamp signal to said second level,
the control means detects the third distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to the third number, and
the control means calculates the distance to the object, based on the sum of said second distance value and said third distance value.
The first number, the second number, and the third number may be equal to each other or different from each other.
According to the first aspect, even if the second distance value is detected as a near-side value under the conditions of the ambient light luminance being relatively high and the object having a large reflectance, when the second distance value is a value nearer than the second set distance, the third distance value is detected with the clamp signal being set at the first level of the high level and the distance to the object is calculated based on the sum of the first distance value and the third distance value. Therefore, the distance to the object can be detected with accuracy.
[2] A second aspect of the distance measuring apparatus according to the present invention is a distance measuring apparatus comprising:
light projecting means for projecting light toward an object to be measured;
light receiving means for receiving reflected light of said light projected toward the object, at a reception position on a position sensitive device according to a distance to said object, and for outputting a far signal, which is a value increasing with increase in said distance if quantity of received light is constant, and a near signal, which is a value increasing with decrease in said distance if the quantity of received light is constant;
clamping means for accepting said far signal and comparing the far signal with a level of a clamp signal, wherein said clamping means outputs said far signal if a level of said far signal is not less than the level of said clamp signal, or outputs said clamp signal otherwise;
arithmetic means for calculating a ratio of said near signal and a signal outputted from said clamping means to output an output ratio signal;
integrating means for cumulatively integrating said output ratio signal and outputting an integral signal according to a result of integration; and
control means for controlling each of projection of said light in said light projecting means, the level of said clamp signal in said clamping means, and the total sum of integral time of said output ratio signal in said integrating means, and for detecting a distance value, based on said integral signal outputted from said integrating means,
wherein said control means works as follows:
(a) the control means sets said clamp signal to a first level, and
the control means detects a first distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to a first number;
(b) when said first distance value is a value farther than a first set distance, the control means sets said clamp signal to a second level smaller than said first level, and
the control means detects a second distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to a second number;
(c) when a difference of said second distance value from said first distance value is smaller than a first set value, or, when the difference of said second distance value from said first distance value is not smaller than the first set value and when said second distance value is a value farther than a third set distance,
the control means sets said clamp signal to said first level,
the control means detects a third distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to a third number, and
the control means calculates the distance to said object, based on the sum of said first distance value and said third distance value;
(d) when the difference of said second distance value from said first distance value is not smaller than the first set value and when said second distance value is not a value farther than the third set distance,
the control means sets said clamp signal to said second level,
the control means detects the third distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to the third number, and
the control means calculates the distance to the object, based on the sum of said second distance value and said third distance value.
The first number, the second number, and the third number may be equal to or different from each other.
According to the second aspect, even if the difference of the second distance value from the first distance value is not smaller than the first set value, when the second distance value is a value farther than the third set distance, the third distance value is detected with the clamp signal being set at the first level and the distance to the object is calculated based on the sum of the first distance value and the third distance value. In this structure, when the reflectance of the object is close to the reference reflectance, the third distance value is detected with the clamp signal being set at the first level whereby the third distance value is prevented from being detected as a value nearer than a designed value. For this reason, the distance to the object can be detected with accuracy.
[3] A third aspect of the distance measuring apparatus according to the present invention is a distance measuring apparatus comprising:
light projecting means for projecting light toward an object to be measured;
light receiving means for receiving reflected light of said light projected toward the object, at a reception position on a position sensitive device according to a distance to said object, and for outputting a far signal, which is a value increasing with increase in said distance if quantity of received light is constant, and a near signal, which is a value increasing with decrease in said distance if the quantity of received light is constant;
clamping means for accepting said far signal and comparing the far signal with a level of a clamp signal, wherein said clamping means outputs said far signal if a level of said far signal is not less than the level of said clamp signal, or outputs said clamp signal otherwise;
arithmetic means for calculating a ratio of said near signal and a signal outputted from said clamping means to output an output ratio signal;
integrating means for cumulatively integrating said output ratio signal and outputting an integral signal according to a result of integration; and
control means for controlling each of projection of said light in said light projecting means, the level of said clamp signal in said clamping means, and the total sum of integral time of said output ratio signal in said integrating means, and for detecting a distance value, based on said integral signal outputted from said integrating means,
wherein said control means works as follows:
(a) the control means sets said clamp signal to a first level, and
the control means detects a first distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to a first number;
(b) when said first distance value is a value farther than a first set distance, the control means sets said clamp signal to a second level smaller than said first level, and
the control means detects a second distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to a second number;
(c) either when the sum of said first distance value and said first distance value is not larger than a second set value or when a difference of said second distance value from said first distance value is not larger than a third set value and when the difference of said second distance value from said first distance value is smaller than a first set value,
the control means sets said clamp signal to said first level,
the control means detects a third distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to a third number, and
the control means calculates the distance to said object, based on the sum of said first distance value and said third distance value;
(d) when the sum of said first distance value and said first distance value is larger than the second set value and when the difference of said second distance value from said first distance value is larger than the third set value, or, either when the sum of said first distance value and said first distance value is not larger than the second set value or when the difference of said second distance value from said first distance value is not larger than the third set value and when the difference of said second distance value from said first distance value is not smaller than the first set value,
the control means sets said clamp signal to said second level,
the control means detects the third distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to the third number, and
the control means calculates the distance to the object, based on the sum of said second distance value and said third distance value.
The first number, the second number, and the third number may be equal to or different from each other.
According to this configurations, when the sum of the first distance value and the first distance value or the like is larger than the second set value and when the difference of the second distance value from the first distance value is larger than the third set value, the third distance value is detected with the clamp signal being set at the second level and the distance to the object is calculated based on the sum of the second distance value and the third distance value. In this structure, when the object has a low reflectance, the third distance value is detected with the clamp signal being set at the first level, which reduces a possibility of detecting the third distance value as a value farther than a designed value. For this reason, the distance to the object can be detected with accuracy.
[4] A fourth aspect of the distance measuring apparatus according to the present invention is a distance measuring apparatus comprising:
light projecting means for projecting light toward an object to be measured;
light receiving means for receiving reflected light of said light projected toward the object, at a reception position on a position sensitive device according to a distance to said object, and for outputting a far signal, which is a value increasing with increase in said distance if quantity of received light is constant, and a near signal, which is a value increasing with decrease in said distance if the quantity of received light is constant;
clamping means for accepting said far signal and comparing the far signal with a level of a clamp signal, wherein said clamping means outputs said far signal if a level of said far signal is not less than the level of said clamp signal, or outputs said clamp signal otherwise;
arithmetic means for calculating a ratio of said near signal and a signal outputted from said clamping means to output an output ratio signal;
integrating means for cumulatively integrating said output ratio signal and outputting an integral signal according to a result of integration; and
control means for controlling each of projection of said light in said light projecting means, the level of said clamp signal in said clamping means, and the total sum of integral time of said output ratio signal in said integrating means, and for detecting a distance value, based on said integral signal outputted from said integrating means,
wherein said control means works as follows:
(a) the control means sets said clamp signal to the first level, and
the control means detects the first distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to the first number;
(b) when said first distance value is a value farther than the first set distance, the control means sets said clamp signal to the second level smaller than said first level, and
the control means detects the second distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to the second number;
(c) either when the sum of said first distance value and said second distance value is not larger than the second set value or when the difference of said second distance value from said first distance value is not larger than the third set value and when the difference of said second distance value from said first distance value is smaller than the first set value,
the control means sets said clamp signal to said first level,
the control means detects the third distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to the third number, and
the control means calculates the distance to said object, based on the sum of said first distance value and said third distance value;
(d) when the sum of said first distance value and said second distance value is larger than the second set value and when the difference of said second distance value from said first distance value is larger than the third set value, or, either when the sum of said first distance value and said second distance value is not larger than the second set value or when the difference of said second distance value from said first distance value is not larger than the third set value and when the difference of said second distance value from said first distance value is not smaller than the first set value,
the control means sets said clamp signal to said second level,
the control means detects the third distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to the third number, and
the control means calculates the distance to the object, based on the sum of said second distance value and said third distance value.
The first number, the second number, and the third number may be equal to or different from each other.
According to this configurations, when the sum of the first distance value and the first distance value or the like is larger than the second set value and when the difference of the second distance value from the first distance value is larger than the third set value, the third distance value is detected with the clamp signal being set at the second level and the distance to the object is calculated based on the sum of the second distance value and the third distance value. In this structure, when the object has a low reflectance, the third distance value is detected with the clamp signal being set at the first level, which reduces a possibility of detecting the third distance value as a value farther than a designed value. For this reason, the distance to the object can be detected with accuracy.
[5] A fifth aspect of the distance measuring apparatus according to the present invention is a distance measuring apparatus comprising:
light projecting means for projecting light toward an object to be measured;
light receiving means for receiving reflected light of said light projected toward the object, at a reception position on a position sensitive device according to a distance to said object, and for outputting a far signal, which is a value increasing with increase in said distance if quantity of received light is constant, and a near signal, which is a value increasing with decrease in said distance if the quantity of received light is constant;
clamping means for accepting said far signal and comparing the far signal with a level of a clamp signal, wherein said clamping means outputs said far signal if a level of said far signal is not less than the level of said clamp signal, or outputs said clamp signal otherwise;
arithmetic means for calculating a ratio of said near signal and a signal outputted from said clamping means to output an output ratio signal;
integrating means for cumulatively integrating said output ratio signal and outputting an integral signal according to a result of integration; and
control means for controlling each of projection of said light in said light projecting means, the level of said clamp signal in said clamping means, and the total sum of integral time of said output ratio signal in said integrating means, and for detecting a distance value, based on said integral signal outputted from said integrating means,
wherein said control means works as follows:
(a) the control means sets said clamp signal to the first level, and
the control means detects the first distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to the first number;
(b) when said first distance value is a value farther than the first set distance, the control means sets said clamp signal to the second level smaller than said first level, and
the control means detects the second distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to the second number;
(c) either when the sum of said second distance value and said second distance value is not larger than the second set value or when the difference of said second distance value from said first distance value is not larger than the third set value and when the difference of said second distance value from said first distance value is smaller than the first set value,
the control means sets said clamp signal to said first level,
the control means detects the third distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to the third number, and
the control means calculates the distance to said object, based on the sum of said first distance value and said third distance value;
(d) when the sum of said second distance value and said second distance value is larger than the second set value and when the difference of said second distance value from said first distance value is larger than the third set value, or, either when the sum of said second distance value and said second distance value is not larger than the second set value or when the difference of said second distance value from said first distance value is not larger than the third set value and when the difference of said second distance value from said first distance value is not smaller than the first set value,
the control means sets said clamp signal to said second level,
the control means detects the third distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to the third number, and
the control means calculates the distance to the object, based on the sum of said second distance value and said third distance value.
The first number, the second number, and the third number may be equal to or different from each other.
According to this configurations, when the sum of the first distance value and the first distance value or the like is larger than the second set value and when the difference of the second distance value from the first distance value is larger than the third set value, the third distance value is detected with the clamp signal being set at the second level and the distance to the object is calculated based on the sum of the second distance value and the third distance value. In this structure, when the object has a low reflectance, the third distance value is detected with the clamp signal being set at the first level, which reduces a possibility of detecting the third distance value as a value farther than a designed value. For this reason, the distance to the object can be detected with accuracy.
[6] A sixth aspect of the distance measuring apparatus according to the present invention is a distance measuring apparatus comprising:
light projecting means for projecting light toward an object to be measured;
light receiving means for receiving reflected light of said light projected toward the object, at a reception position on a position sensitive device according to a distance to said object, and for outputting a far signal, which is a value increasing with increase in said distance if quantity of received light is constant, and a near signal, which is a value increasing with decrease in said distance if quantity of received light is constant;
clamping means for accepting said far signal and comparing the far signal with a level of a clamp signal, wherein said clamping means outputs said far signal if a level of said far signal is not less than the level of said clamp signal, or outputs said clamp signal otherwise;
arithmetic means for calculating a ratio of said near signal and a signal outputted from said clamping means to output an output ratio signal;
integrating means for cumulatively integrating said output ratio signal in an integrating capacitor and outputting an integral signal according to a result of integration; and
control means for controlling each of projection of said light in said light projecting means, the level of said clamp signal in said clamping means, and the total sum of integral time of said output ratio signal in said integrating means, and for detecting a distance value, based on said integral signal outputted from said integrating means,
wherein said control means works as follows:
(a) the control means sets said clamp signal to a first level, and
the control means detects a first distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to a first number;
(b) the control means sets said clamp signal to a second level, and
the control means detects a second distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to a second number smaller than said first number;
(c) the control means corrects said second distance value by adding a predetermined value to said second distance value, and
the control means calculates the distance to said object, based on the sum of said first distance value and said corrected second distance value.
A further aspect of this apparatus is preferably the apparatus wherein said second level of said clamp signal in detection of said second distance value is equal to or smaller than said first level of said clamp signal in detection of said first distance value.
[7] A seventh aspect of the distance measuring apparatus according to the present invention is a distance measuring apparatus comprising:
light projecting means for projecting light toward an object to be measured;
light receiving means for receiving reflected light of said light projected toward the object, at a reception position on a position sensitive device according to a distance to said object, and for outputting a far signal, which is a value increasing with increase in said distance if quantity of received light is constant, and a near signal, which is a value increasing with decrease in said distance if the quantity of received light is constant;
clamping means for accepting said far signal and comparing the far signal with a level of a clamp signal, wherein said clamping means outputs said far signal if a level of said far signal is not less than the level of said clamp signal, or outputs said clamp signal otherwise;
arithmetic means for calculating a ratio of said near signal and a signal outputted from said clamping means to output an output ratio signal;
integrating means for cumulatively integrating said output ratio signal in an integrating capacitor and outputting an integral signal according to a result of integration; and
control means for controlling each of projection of said light in said light projecting means, the level of said clamp signal in said clamping means, and the total sum of integral time of said output ratio signal in said integrating means, and for detecting a distance value, based on said integral signal outputted from said integrating means,
wherein said control means works as follows:
(a) the control means sets said clamp signal to a first level, and
the control means detects a first distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to a first number;
(b) the control means sets said clamp signal to a second level, and
the control means detects a second distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to a second number smaller than said first number;
(c) the control means corrects said second distance value by adding a predetermined value to said second distance value;
(d) the control means sets said clamp signal to a third level, and
the control means detects a third distance value, based on said integral signal outputted from said integrating means while the total sum of integral time of said output ratio signal in said integrating means is set to a third number smaller than said first number;
(e) the control means corrects said third distance value by adding a predetermined value to said third distance value;
(f) the control means calculates the distance to said object, based on the sum of said first distance value and said corrected second distance value or based on the sum of said second distance value and said corrected third distance value.
In this case, the second number and the third number are desirably equal to each other.
A further aspect of this apparatus is preferably the apparatus wherein said second level of said clamp signal in detection of said second distance value is equal to or smaller than said first level of said clamp signal in detection of said first distance value, and
wherein said third level of said clamp signal in detection of said third distance value is equal to or smaller than said first level of said clamp signal in detection of said first distance value.
A further aspect of this apparatus is preferably the apparatus wherein said predetermined value added for correction of said second distance value is equal to said predetermined value added for correction of said third distance value.
According to these configurations, when continuous distance measurement is carried out to detect the distance to the object, based on a plurality of distance values, there occurs change in characteristics of the integrating capacitor between the integral processing for detection of the first distance value and in the integral processing for detection of the second distance value. However, variation of the second distance value due to the change in the characteristics of the integrating capacitor can be corrected in such a way that the total sum of integral time in the detection of the first distance value is made different from that in the detection of the second distance value and that correction is made by adding a predetermined value to the second distance value. For this reason, the distance to the object can be detected with accuracy.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.